1. Field of the Invention
This invention relates generally to a power transmission belt having improved tensile cord, more particularly to a power transmission belt having low shrinkage tensile cord comprising dimensionally stable polyester fibers, and specifically to a V-belt for variable speed or clutching applications having a tensile cord of dimensionally stable polyester fibers sold under the trade name A360 or A363 by Performance Fibers, Inc.
2. Description of the Prior Art
Many cords of different materials for forming the tensile cord of power transmission belts are known. The need for dimensional stability is known for toothed belt applications, and many proposals for achieving dimensional stability exist. One aspect of dimensional stability is low elongation during use, i.e. low growth or high modulus. Another aspect of dimensional stability is heat shrinkage after the belt is warmed up due to running. Dimensional instability problems are normally associated with cord made of thermoplastic polymers such as polyester (such as polyethylene terephthalate or “PET”), nylon, polyvinyl-alcohol, and polyethylene naphthalate (“PEN”). A “Dimensional Stability Index” (“DSI”) may be defined as the sum of percent elongation at 45N per 1000 denier (“d”) (4.5 g/d) plus the percent free heat shrinkage in air at a given temperature such as 177° C. (according to the method of ASTM D885). Examples which address one or more issues related to dimensional stability of toothed belts include U.S. Pat. No. 3,992,959 to Cicognani, U.S. Pat. No. 6,695,733 to Knutson, U.S. Pat. No. 6,358,171 to Whitfield, and U.S. Pat. No. 5,807,194 to Knutson et al. Many of these proposals include very expensive materials such as PBO, carbon fiber, aramid or polyethylenenaphthalate (PEN). Other cord materials proposed for such applications include fiber glass cord, which requires complicated treating processes, and rayon, which has relatively low tensile strength though still more expensive than PET. Thus, PET is generally used where a good, low-cost cord is needed, but where either high elongation or high heat shrinkage can be tolerated. Applications requiring more dimensional stability, i.e. both low elongation and low heat shrinkage, generally require rayon, aramid, glass, carbon, or PEN, PBO, PVA, all of which are higher-performance but also higher in cost. However, many belt applications which have increasing demands for dimensional stability are high-volume applications which require more economical approaches than these proposed materials.
In particular, variable speed applications such as scooters, snowmobiles, and the like place extreme performance requirements on V-belts in general, and on the tensile cord materials in particular. One problem is that elongation and/or heat shrinkage of the belt during use changes the shifting and other performance characteristics of the variable speed drive. As another example, v-belt drives with a clutching engagement mechanism as used in power tools, lawn mowers, and the like require dimensional stability for proper clutch operation, as described for example in U.S. Pat. No. 6,595,883 to Breed et al. As a third example, timing belts or toothed belts or synchronous belts also require excellent dimensional stability to maintain proper meshing with associated toothed pulleys or sprockets. While laundry lists of cord materials for such belts often include materials such as polyester, rayon, or nylon, it is generally recommended that higher modulus and higher strength and higher cost materials such as aramid, carbon, glass, be used in such applications demanding extreme dimensional stability. Polyester in particular is economical, and can be stretched during processing to provide high modulus and low elongation, but then has excessive heat shrinkage, which may be useful for maintaining tension in some belt drives, but is detrimental for variable speed drives requiring dimensional stability. Thus, improved polyesters, including the so-called dimensionally stable polyester (“DSP”) or high-modulus, low-shrinkage polyester (“HMLS” polyester) are economical but still exhibit insufficient dimensional stability for some applications. An example of a DSP having a DSI of 8% to 11% is disclosed in U.S. Pat. No. 5,067,538. A cord with DSI less than about 6% is not disclosed or suggested. The need remains for a more economical cord material with suitable high performance and dimensional stability characteristics for v-belts for variable speed applications, including clutching applications, and the like.